The present invention relates to electrodes, particularly to carbon electrodes, and more particularly to the fabrication of solid carbon porous electrodes from powders, with the use thereof being determined by the surface area thereof and dopant therein.
Carbon is an attractive candidate for use in various types of electrodes including lithium-intercalation negative electrodes for lithium ion or rechargeable cells because of its low cost, chemical stability, and excellent reversibility for lithium (Li) insertion. Cells containing Li-intercalated carbon electrodes are more easily fabricated than similar cells containing lithium metal because the carbon electrode components are stable in air. A variety of carbonaceous materials (graphite, petroleum coke, carbon black, carbon fiber, glassy carbon, etc.), having a wide spectrum of physical and chemical characteristics, have been evaluated in Li-intercalation electrodes. However, the factors which influence the electrode capacity, irreversible capacity loss, voltage profile and cycle life are not fully understood. The variety of commercial carbonaceous materials with different particle morphologies and structures, varying composition of carbon and other elements, and different surface properties require customized electrode fabrication procedures. These varying techniques and complexed electrode compositions, makes analysis and correlation of the electrochemical results particularly difficult. However, the availability and low cost of these materials make them particularly attractive for electrode fabrication.
During the evaluation efforts that lead to the present invention, both commercial and polymer-derived carbons were evaluated for various electrode applications. The carbons and graphites obtained from commercial sources were divided into to classes (graphitized or non-graphitized) based on the information provided from the manufacturers and/or their pyrolysis temperature (Tp&lt;1350.degree. C. considered non-graphitized). This distinction was made because a different electrolyte was used to evaluate the graphitized and non-graphitized carbon. For further details of these evaluations see paper (UCRL-JC-117265), T. D. Tran et al, "Carbonaceous Materials As Lithium Intercalation Anodes", presented at The 186th Meeting of the Electrochemical Society, Miami Beach, Fla., October 1994, published March 1995.
In addition to the carbons available from commercial sources, several polymer-based carbons were also synthesized and evaluated. The polymer-based carbons were obtained by carbonizing polyacrylonitrile (PAN), manufactured by DuPont, phenolic resin, manufactured by Reichold Chemical, and polyfurfuryl alcohol (QO Chemicals) at 1050.degree. C. for 3 hours in a nitrogen atmosphere. The polyfurfuryl alcohol was obtained from the phosphoric acid-catalyzed polymerization of furfuryl alcohol. These carbonized materials were ground and sieved to between 30-60 .mu.m before testing in an electrode structure.
The influence of a phosphorous additive on the ability of carbon to intercalate Li was also investigated, since many of the carbonaceous materials previously used as lithium intercalation anodes in lithium-ion or rocking-chair rechargeable cells have capacity below the theoretical value of 372 mAh/g which corresponds to 1 mole of lithium per 6 moles of carbon (LiC.sub.6). It was found that phosphorous in carbonaceous materials, such as coke, enhances its capacity for Li intercalation. It was found that by doping the above-references polymer-based carbons enhanced the electrode capacity. These evaluation efforts involved the influence of a phosphorus additive on the ability of monolithic polyacrylonitrile-derived porous carbon forms, to intercalate lithium, the results of which are described in a paper UCRL-JC-119221 by T. D. Tran et al, "Lithium Intercalation In Porous Carbon Anodes", 1994 Fall Meeting of the Material Research Society Nov. 28-Dec. 2, 1994.
Prior studies have been conducted on binders for the various types of carbon and graphite, and it has been determined that the choice of binder can have a major influence on the electrochemical performance of carbon for Li intercalation. During the above-referenced evaluation of the polymer-based carbons, binder evaluation was carried out, and the experiments indicated that both the irreversible capacity loss on the first cycle and the reversible capacity for lithium intercalation were dependent on the type of binder used in the carbon electrode. These evaluations are also set forth in T. D. Tran et al, "Carbonaceous Materials As Lithium Intercalation Anodes", referenced above.
From the above-referenced evaluations of various carbonaceous materials it was recognized that a need existed for a method of forming solid porous electrodes from inexpensive materials. That need has been satisfied by the present invention which broadly involves the formation of solid carbon porous electrodes from powders of graphites, pyrolyzed cokes, carbons, polymer-desired carbons, and carbon aerogels. In accordance with the present invention, solid porous electrodes have been produced by combining powders of various carbon blacks, examples of which include pyrolyzed cokes, powders derived from the pyrolysis of polyfurfuryl alcohol, polyacrylonitrile, carbon aerogel microspheres and powders, and various graphite powders.